It is known that when an alternating electric field is applied, a piezoelectric crystal mechanically vibrates at a particular frequency. Then, even after application of the alternating electric field is stopped, the mechanical vibration is maintained with attenuation by a piezoelectric effect and thereby generates a new alternating electric field.
Thus, by detecting the newly generated alternating electric field as a piezoelectric effect signal with an electric field probe (an electric field type antenna), the presence or absence of a piezoelectric crystal can be detected in a non-contact (e.g., Non-patent Document 1).
At that time, in a case that an electric field probe having a function of both transmission and reception capable of also applying an alternating electric field to the piezoelectric crystal is employed as the electric field probe, detection of a piezoelectric crystal can efficiently be achieved.
Such piezoelectric crystals include illicit drugs such as cocaine and stimulant. Then, as a specific example that an electric field probe having a function of both transmission and reception is applied to detection of an illicit drug, a technique is proposed that: a capacitor constructed from two metal plates is employed and then an object to be inspected such as a bag is inserted between the metal plates; and then an alternating electric field is applied between the metal plates and thereby a piezoelectric effect signal from an illicit drug such as cocaine and stimulant is received so that a concealed illicit drug is detected in a sealed state in a non-contact (e.g., Patent Document 1).